Williamson played under Appleton at Portsmouth earlier this season but his Rovers debut against Arsenal in the FA Cup on Saturday was his first action in six weeks since leaving the stricken south-coast club.
Appleton hailed the 30-year-old's composed use of possession when the visitors were under heavy pressure before triumphing 1-0 at the Emirates Stadium, although his weekend exertions may mean he forgoes a starting place in Tuesday's Championship trip to Hull.
"He wasn't one of the big-name ones that came through in terms of what we were trying to do before the (transfer) window shut but he's got a lot of quality," Appleton said.
"I thought he was magnificent on Saturday. In the first half where we lacked a little bit of composure at times when we won the ball back, he was the only one who steadied the game down a little bit and played to his pace.
"I think a lot of the players took that on board at half-time and when we won the ball back in the second half we were a lot more composed and we played an extra pass when we needed to.
"I'm delighted with Lee. He's not played for six weeks. He was a little bit sore and stiff on Sunday but he's going to be a good addition for the football club."